It is a general requirement to design a radio frequency power amplifier such that the radio frequency power amplifier is highly efficient and is substantially insensitive to load impedance variation and component variation or mismatching load impedances. Power efficiency, for example, is expressed as the power added efficiency (PAE) that takes into account the power Pin of an input signal. The power efficiency ratio is defined as the ratio of the difference of the output signal power (Pout) and input signal power (Pin) to the DC power consumed:PAE=(Pout−Pin)/PDC=(Pout−Pin)/(VDC*IDC)
In this formula VDC is the supply voltage of the amplifier and IDC is the consumed supply current of the amplifier.
The power efficiency can be relatively easy optimized for a single specific output level and a specific frequency. However, modern mobile communication networks transmit radio signals at different power levels and at different frequencies in order to improve the overall network interference. In addition to the different power levels the use of efficient modulation schemes leads to relatively high peak-to-average ratios (PAR) of the radio signal. The amplifiers with the high power efficiency for high peak-to-average ratio signals therefore should provide a flat gain versus power characteristic over a wide frequency range.
So far at least two basic approaches are known to tackle the above-mentioned issues. Doherty amplifiers have a relatively simple structure but operate only in a very narrow frequency band and are very sensitive to component variation. It is difficult to tune both load impedances of the Doherty amplifier and it is further difficult to achieve a flat gain versus power characteristic. In contrast, radio frequency amplifiers with envelope tracking cope well with component and load variations. However, the instantaneous bandwidth of the radio frequency amplifier with envelope tracking is too low and switching supply voltages create spurs.
Operators of the mobile communications networks have increased the number of base transceiver stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to reduce the purchase costs as well as the running costs of the base transceiver stations. Therefore there is a need for radio frequency power amplifiers with high efficiency, in order to reduce the power consumption and at the same time to keep the manufacturing costs for such radio frequency power amplifiers low to be able to sell the radio frequency power amplifiers at prices that are acceptable by the customer.